A flexible metal-clad laminate used in manufacturing a flexible printed circuit board, which is a laminate of a conductive metal clad and an insulating resin, may be subject to a micro-circuit processing and may be bent in a narrow space. Therefore, the flexible metal-clad laminate has been increasingly utilized for a notebook computer, a personal digital assistant, a miniaturized video camera, a storage disk, and the like, in accordance with the trend in which an electronic device has a small size and a light weight. It is general that the flexible metal-clad laminate has a two layered structure or a three layered structure.
In the case of the flexible metal-clad laminate having the three layered structure, a polyimide film is coupled with the metal-clad by using an epoxy-based or a urethane-based adhesive. In this case, heat resistant property and flame retardant property are deteriorated due to the adhesive layer, and since change in dimension during an etching process and a heat treatment process is large, problems may occur in manufacturing a printed circuit board. In order to overcome the above-described problem, a flexible metal-clad laminate having a two layered structure has been developed only by using thermoplastic polyimide and polyimide without using the adhesive and has been used.
The flexible metal-clad laminate having the two layered structure may be largely classified into a single-sided metal-clad laminate having the metal-clad and the polyimide film and a double-sided metal-clad laminate having the polyimide film between two layered metal-clads. Here, the polyimide film is generally formed of a multi-layer polyimide having two or more layers rather than a single layer, wherein the multi-layer polyimide includes polyimide having different coefficient of linear thermal expansion in order to satisfy properties such as adhesion with the metal-clad, dimensional stability, and the like. In the flexible metal-clad laminate including the multi-layer polyimide film having two or more layers, warpage and a curl between the metal-clad and the polyimide laminate may be prevented, and physical properties such as adhesion strength, mechanical physical property, and electrical property may be improved.
Korean Laid-Open Patent Publication No. 10-2012-0123389 (Patent Document 1) discloses a flexible metal laminated board using a multi-layer polyimide film and having a thermoplastic polyimide layer formed on at least one side of a non-thermoplastic polyimide layer.
In order to generally form the multi-layer polyimide film used in the flexible metal-clad laminate, there are a method that a thermoplastic polyamic acid is applied and dried onto the previously manufactured polyimide film, followed by heating at a high temperature, and a method that a polyamic acid is applied and dried onto the metal-clad as many as the number of layers to be laminated, followed by heating at a high temperature.
In the case of using the above-described methods, since a thermosetting is partially performed on a surface of a polyimide precursor layer in a drying process, upper and lower polyimide precursor solutions are not mixed in an interface but each interlayer interface is clearly differentiated in the case of applying another polyimide precursor solution on the surface of the dried polyimide precursor layer. In this case, an inner turbidity of the polyimide film is increased and a light transmittance is hindered due to the reflection at the interlayer interface of the polyimide.
Recently, a scheme that a light source is used to penetrate the polyimide film through the board, thereby confirming a bonding position and coupling with the flexible circuit board and a module has been used in coupling the flexible circuit board and the module, such that in the case of using the polyimide having a low light transmittance, time required for recognizing the bonding position may be increased and a bonding defect between the flexible circuit board and the module may occur after the bonding, that is, a yield may be deteriorated. Therefore, a metal-clad laminate in which a light reflection on the surface of the polyimide film and a light reflection generated at the interlayer interface of the polyimide in the multi-layer polyimide film are remarkably decreased to have an improved light transmittance after the metal-clad is removed has been demanded.
(Patent Document 1) Korean Laid-Open Patent Publication No. 10-2012-0123389